A Raman optical amplifier is an optical amplifier based on a Raman effect. The Raman effect means that when a beam of weak light and a beam of intense light enter a fiber simultaneously (in a same direction or in opposite directions), if a spectrum of the weak light just falls within a range of a Raman gain spectrum of the intense light, energy of the intense light is transferred to the weak light, so that the weak light is amplified. Therefore, in a communications network, signal light may be amplified based on the Raman effect, thereby increasing a transmission distance of the signal light.
During network running, because optical channel adding or dropping is caused by grooming, fiber cut, or the like, a distribution of transmission channels changes. For example, a quantity of transmission channels increases or decreases. For another example, a quantity of transmission channels does not change, but a distribution of the transmission channels in spectral space changes. The change in the distribution of the transmission channels causes a change in a gain of a Raman optical amplifier. In addition, interference between circuits or the like may lead to a change in a drive current or drive voltage of a pump source, and may further lead to a change in a pump light power, which may also lead to a change in the gain of the Raman optical amplifier. However, such changes of gains of output signal light may be accumulated in a concatenation link, and may cause a link fault. Therefore, such changes of gains need to be monitored in the communications network.
However, in the prior art, the change in the gain of the Raman optical amplifier can hardly be determined accurately for various reasons, and consequently, a communications network fault may be caused.